Wanting to make frequently-performed tasks possible with just one hand instead of two, I tried mapping inventory actions like "Stack split" to various mouse buttons but these mappings are apparently not being honored in-game.

As a concrete example (steps to reproduce):

Clear any existing mapping for the input "Middle Mouse Button"

Clear any existing mapping for the action "Stack split"

Map the "Stack split" action to the "Middle Mouse Button" input

Expected result:
When viewing the contents of a chest, middle-clicking on a stack in your inventory will transfer half the stack to the chest.